Certain rail vehicles (e.g., locomotives) and other off-highway vehicles are powered by electric traction motors coupled in driving relationship to one or more axles of the vehicle. In a motoring mode of operation, the traction motors are supplied with electric current from a controllable source of electric power such as an engine-driven traction alternator. The traction motors apply torque to the vehicle wheels, which in turn exert tangential force (tractive effort) on the surface on which the vehicle is traveling, e.g., the parallel steel rails of a railroad track, and thereby propel the vehicle in a desired direction along a route of travel. In another instance, in a dynamic braking mode of operation, the motors serve as axle-driven electrical generators. In this mode of operation, the traction motors exert torque in an opposite direction from the rolling direction of the wheels, thereby slowing the vehicle's motion. In either case, good adhesion between each wheel and the surface facilitates efficient operation of the vehicle.
Maximum tractive effort or braking effort is obtained if each powered wheel of the vehicle is rotating at such an angular velocity that its actual peripheral speed (e.g., wheel speed) is slightly higher (in case of motoring) or slightly lower (in case of braking) than the actual speed of the vehicle. The difference between the linear speed at which the vehicle is traveling (referred to as ground speed) and wheel speed is referred to as slip speed, creep, or wheel creep. There is usually a relatively low limit on the value of slip speed at which peak tractive effort or braking effort is realized. This value, commonly known as optimum creep, is a variable that depends on ground speed and travel surface conditions. Operation of any or all wheels away from the optimum creep, e.g., at too small a creep value or too large a creep value, may result in a reduction or loss of wheel-to-surface adhesion. Likewise, if the wheel-to-surface adhesion tends to be reduced or lost, some or all the vehicle wheels may slip excessively, i.e., the actual slip speed or creep may be greater than the optimum creep. Such a wheel slip condition, which is characterized in the motoring mode by one or more slipping axle-wheel sets and in the braking mode by one or more sliding or skidding axle-wheel sets, can cause accelerated wheel wear, rail damage, high mechanical stresses in the drive components of the propulsion system, and an undesirable decrease of tractive (or braking) effort.
Wheel creep may be controlled using creep regulators. However, especially at low speeds, creep regulators have an increasingly more difficult time regulating wheel speeds. This leads to large overshoots in wheel speed, rapid torque reductions, and torque reapplication limit cycles. This may result in high wheel creep transients capable of damaging the rail head (in the case of a rail vehicle), reduced tractive effort, rapid suspension displacements, poor vehicle ride quality, and vehicle reference speed errors.